Extension Type,Description,Reference
0x0000-0x0011,Unassigned,
0x0012,Multi-Prefix,[RFC5535]
0x0013-0xFFFC,Unassigned,
0xFFFD,Exp_FFFD (experimental),[RFC4581]
0xFFFE,Exp_FFFE (experimental),[RFC4581]
0xFFFF,Exp_FFFF (experimental),[RFC4581]